Trunk piston engines may be used in marine, power-generation and rail traction applications and have a higher speed than cross-head engines. A single lubricant (TPEO) is used for crankcase and cylinder lubrication. All major moving parts of the engine, i.e. the main and big end bearings, camshaft and valve gear, are lubricated by means of a pumped circulation system. The cylinder liners are lubricated partially by splash lubrication and partially by oil from the circulation systems that finds its way to the cylinder wall through holes in the piston skirt via the connecting rod and gudgeon pin. Trunk piston engines normally include a centrifuge to clean the TPEO.
Zinc dialkyl dithiophosphates (“ZDDP's”) are known in the art as additives for TPEO's to provide wear protection for gears and valve train in trunk piston engines. However, the presence of water may destabilise the ZDDP molecule leading to depletion of phosphorus, the key element for provision of wear protection. Some ZDDP's may reduce phosphorus depletion but at the expense of FZG wear performance.
A problem in the art is therefore to provide ZDDP's in TPEO's that constitute a good balance between reducing phosphorus depletion in the presence of water and FZG wear performance.